For operating conditions as extreme as these, the technologies currently used for pressure measurement are very limited and are based on the use of macroscopic mechanical parts made from high-strength steel alloys such as inconel or quartz and are associated with complex manufacturing techniques. Consequently, the high-pressure probes available on the market are large (typically several tens of centimeters long), expensive and offer only limited features.
An important characteristic of downhole instrumentation requirements is due to the fact that most phenomena to be assessed during the drilling, production or stimulation operations may be related to small variations around the nominal pressure value which, as described previously, is very high.
Recent breakthroughs in the silicon and microelectronics industry have led to the development of miniaturised pressure sensors. The main advantage of this technology is that the sensitive elements are manufactured in batches from silicon wafers, in order to produce a large number of “chips” at low cost.
However, the performance and reliability of sensors using sensitive elements of this type are limited, especially under the above-mentioned extreme pressure conditions.
Silicon is in fact a very brittle material when it is subjected to elongation/strain forces. A relative elongation/strain of about 1% breaks the crystal and therefore destroys the sensor. Even for tension stress levels less than this limiting value, the dislocations present in the crystal shift, resulting in fatigue problems and damage to the sensitive element.
The situation is different when the silicon is subjected to compression forces where, in this case, the material can withstand very high stresses without fatigue problems. In practice, contractions of up to about 5% can be accepted without risk of breakage.
When developing a sensor, it is therefore critical to optimise the distribution of stresses in the structures. This distribution will define the sensor's metrological performance, sensitivity, stability and robustness.
The sensitive element of a silicon pressure sensor according to the state of the art is shown on FIGS. 1A and 1B and comprises a rigid frame 1 with at its centre an area of reduced thickness created by micromachining the silicon in order to create a sensitive membrane 2 comprising an upper wall 3 and a lower wall 4.
Stress-sensitive resistors 5, including two resistors 5a positioned longitudinally and two resistors 5b positioned transversally, also called piezoresistors or gauges, are located on the membrane 2 and connected together by connection means 6 in order to form a Wheatstone bridge measurement circuit.
We refer to FIG. 2, illustrating the sensitive element of the sensor which has a plate 8 attached under its lower side, thereby forming a cavity 9 under vacuum and creating an absolute pressure sensor.
The sensitive element, known in the prior art, operates as follows: the effect of the pressure applied on the upper wall 3 of the membrane 2 creates a force which induces a deflection in said membrane and the appearance of mechanical stresses in the plane of the membrane, which are measured by the piezoresistors.
FIG. 3 illustrates the shape taken by the stress CT along axis AM of the membrane for a given pressure.
The areas of maximum stress appear at the junction areas (in tension MT on the upper wall of the membrane and in compression MC on its lower wall) and at the centre of the membrane (in compression MC on the upper wall and in tension MT on the lower wall).
FIG. 4 is a diagrammatic representation of the stress detection circuit according to the prior art.
The resistors are supplied with voltage and current from the exterior via electrical contacts 7a and 7b. 
The output of the sensitive element is defined by the voltage difference Vs between contacts 7c and 7d which is expressed by the following relations obtained in reference to FIG. 5:Vs=½. V.(ΔRl/Rl−ΔRt/Rt),ΔRl/Rl=Gl×Δl/l, ΔRt/Rt=Gt×Δl/l, in which:                V is the bridge power supply voltage,        Rl is the value of a longitudinal piezoresistor 5a (the two longitudinal gauges are assumed to be identical), which is subjected to an elongation Δl/l due to the effect of pressure on the membrane which is directed along the current flow axis in the resistor, and ΔRl the variation in the value of this resistor under the effect of this stress with respect to the membrane position at rest, i.e. without pressure.        Rt is the value of a transverse piezoresistor 5b (the two transverse gauges are assumed to be identical), which is subjected to an elongation Δl/l due to the effect of pressure on the membrane which is directed perpendicular to the current flow axis in the resistor, and ΔRt the variation in the value of this resistor under the effect of this stress with respect to the membrane position at rest (without pressure).        Gl and Gt are the longitudinal and transverse gauge factors of the piezoresistors. For monocrystalline silicon, these factors depend on the orientation, type and doping concentration (e.g. the boron doping concentration CB in atoms per cm3) and the temperature T (in degrees Celsius), as illustrated on FIGS. 6 and 7.        
The arrow labelled i represents the current direction.
This type of sensitive element exhibits the following disadvantages, which downgrade the sensor performance:                the area of maximum stress is concentrated on a small area and must be perfectly aligned with respect to the positions of the junction areas of the membrane, making it difficult to position the piezoresistors and resulting in a loss of signal;        the maximum allowable pressure of the sensor is limited by the fact that silicon is subjected to a high level of tension stress at the junction areas. Consequently, the sensitivity must be limited to remain below the breakage and stability stress levels.        